No, you certainly should not convert your existing mp3's.
Can I run it on a folder?
You have been give the link in the webpage of Pjotr (link provided by Kadaitcha Man), but be careful. That is only suitable for a random collection of mp3 files. It will search the current folder *and* all subfolders, and apply individual correction to all mp3 files it finds.
You do not want that for albums: you do not want the relative volume be changed between tracks. For albums, you indeed want to do the correction only for the files of the album within a single folder, and with the "-a" option instead of the "-r" option. Then the command is simply
If the single folder contains unrelated music files, you can use the -r ("radio") option. This adjusts the volume of each file independently, whereas the -a ("album") option calculates the overall volume for all files together, and adjusts all files the same way.
[/quote]Is it a permanent change or can I change it again?[/quote]
To some extent the change can be reverted: with the "-r" option, mp3gain changes the actual audio data, but it includes an undo tag that indicates how to restore the frames to their initial volume. As long as the tag is not lost, the change can be undone. Without the "-r" option, i.e., using the system of replaygain tags, nothing happens to the actual audio data.
To undo, use the -u (undo) option, as in
